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Abstract:

This disclosure relates to methods of making a cathode for a lithium
batter. The batterys include: (a) treating a cathode current collector
with flame or corona; (b) coating a slurry containing iron disulfide, a
first solvent, and a binder onto the cathode current collector obtained
from step (a) to form a coated cathode current collector, in which the
slurry contains about 73-75% by weight solids and the binder contains a
polymer selected from the group consisting of linear di- and tri-block
copolymers, linear tri-block copolymers cross-linked with melamine resin,
ethylene-propylene copolymers, ethylene-propylene-diene terpolymers,
tri-block fluorinated thermoplastics, hydrogenated nitrile rubbers,
fluoro-ethylene-vinyl ether copolymers, thermoplastic polyurethanes,
thermoplastic olefins, and polyvinylidene fluoride homopolymers; and (c)
drying the coated cathode current collector obtained from step (b) to
provide a cathode, in which the cathode contains no more than 0.5% by
volume of the first solvent and is capable of being bent to 180°.
This disclosure also relates to methods of making a lithium battery.

3. The battery of claim 2, wherein the binder comprises a
styrene-ethylene-butylene-styrene polymer.

4. The battery of claim 1, wherein the cathode current collector
comprises aluminum or an aluminum alloy.

5. The battery of claim 1, wherein the cathode current collector
comprises an aluminum foil.

6. The battery of claim 1, wherein the solvent comprises a hydrocarbon
solvent.

7. The battery of claim 6, wherein the hydrocarbon solvent is an
unbranched hydrocarbon.

8. The battery of claim 6, wherein the hydrocarbon solvent is a branched
hydrocarbon.

9. The battery of claim 6, wherein the hydrocarbon solvent is an aromatic
hydrocarbon.

10. The battery of claim 1, wherein the solvent comprises a paraffinic
solvent.

11. The battery of claim 10, wherein the paraffinic solvent is an
iso-paraffinic solvent.

12. The battery of claim 10, wherein the paraffinic solvent is a
cyclo-paraffinic solvent.

13. The battery of claim 1, wherein the cathode has a thickness of from
16 mil to 22 mil.

14. The battery of claim 1, wherein the cathode further comprises a
conductive material selected from the group consisting of carbon black
and graphite.

15. The battery of claim 1, wherein the cathode has a porosity of from
30% to 35%.

16. The battery of claim 1, wherein the cathode comprises less than about
3% by weight of the binder.

17. The battery of claim 1, wherein the cathode is capable of bending
180.degree..

18. A method of using a primary lithium battery, the battery comprising a
housing and within the housing (a) an anode comprising lithium (b) a
cathode comprising a cathode current collector and a uniform and defect
free coating comprising iron disulfide and a binder selected from the
group consisting of linear di- and tri-block copolymers, linear tri-block
copolymers cross-linked with melamine resin, ethylene-propylene
copolymers, ethylene-propylene-diene terpolymers, tri-block fluorinated
thermoplastics, hydrogenated nitrile rubbers, fluoro-ethylene-vinyl ether
copolymers, thermoplastic polyurethanes, thermoplastic olefins, and
polyvinylidene fluoride homopolymers and (c) an electrolyte consisting of
(i) dimethyloxyethylene, dioxolane and optionally other solvents, and
(ii) lithium iodide and optionally other salts, the method comprising
discharging the battery only once and then discarding the battery.

19. The method of claim 18, wherein the binder comprises a
styrene-ethylene-butylene-styrene polymer.

20. The battery of claim 18, wherein the cathode current collector
comprises aluminum or an aluminum alloy.

21. The battery of claim 18, wherein the cathode further comprises a
conductive material selected from the group consisting of carbon black
and graphite.

22. The battery of claim 18, wherein the cathode has a porosity of from
30% to 35%.

23. The battery of claim 18, wherein the cathode comprises less than
about 3% by weight of the binder.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of and claims the benefit of
priority under 35 U.S.C. §120 to U.S. application Ser. No.
12/791,221, filed on Jun. 1, 2010, which is a continuation of U.S.
application Ser. No. 12/408,869, filed on Mar. 23, 2009, now U.S. Pat.
No. 7,753,968, which is a continuation and claims the benefit of priority
under 35 U.S.C. §120 to U.S. application Ser. No. 11/375,537, filed
on Mar. 14, 2006, now U.S. Pat. No. 7,527,895, which is a continuation of
and claims the benefit of priority under 35 U.S.C. §120 of U.S.
application Ser. No. 10/290,832, filed on Nov. 8, 2002, now U.S. Pat. No.
7,033,698. The contents of all parent applications are hereby
incorporated by reference in their entireties.

BACKGROUND

[0002] This invention relates to cathodes for lithium batteries.

[0003] Batteries are commonly used electrical energy sources. A battery
contains a negative electrode, typically called the anode, and a positive
electrode, typically called the cathode. The anode contains an active
material that can be oxidized; the cathode contains or consumes an active
material that can be reduced. The anode active material is capable of
reducing the cathode active material.

[0004] When a battery is used as an electrical energy source in a device,
electrical contact is made to the anode and the cathode, allowing
electrons to flow through the device and permitting the respective
oxidation and reduction reactions to occur to provide electrical power.
An electrolyte in contact with the anode and the cathode contains ions
that flow through the separator between the electrodes to maintain charge
balance throughout the battery during discharge.

[0005] The cathode of the battery can be prepared by applying a slurry
containing an active material to a substrate, which can serve as the
current collector for the cathode. It is desirable to coat the substrate
uniformly, because a uniform coating thickness can promote good battery
performance. Certain extrusion processes offer good control of the
coating thickness, but cannot be used when the slurry contains materials
that fibrillate, and thus become rigid, during the extrusion process.

SUMMARY

[0006] The invention relates to methods for making cathodes for lithium
batteries, and to the cathodes produced by these methods. The batterys
include forming a slurry containing active materials, a binder, and a
solvent or solvents, coating a flexible current collector with the
slurry, then drying and calendering the cathode.

[0007] The finished cathodes are very thin and very flexible. Single-sided
cathodes (i.e., cathodes in which only one side of the foil is coated
with active materials) can be folded 180° to form pleats, or can
be wound on small-diameter round or square mandrels, without any cracking
or delamination of the coating. Double sided cathodes can be wound as
well, without cracking or delamination of the coating. The cathodes are
comparable in performance to cathodes made using stainless steel mesh
current collectors. The thinness of the current collector allows for the
use of increased amounts of active material per volume.

[0008] In one aspect, the invention features a cathode for a lithium
battery. The cathode includes: (a) a current collector including aluminum
foil; and (b) active cathode material including: (i) manganese dioxide;
(ii) conductive materials; and (iii) a binder selected from the group
consisting of linear di- and tri-block polymers, linear tri-block
polymers cross-linked with melamine resin, ethylene-propylene copolymers,
ethylene-propylene-diene terpolymers, tri-block fluorinated
thermoplastics, hydrogenated nitrile rubbers, fluoro-ethylene-vinyl ether
copolymers, thermoplastic polyurethanes, thermoplastic olefins, and
polyvinylidine fluoride homopolymers. The binder may be a tri-block
copolymer, e.g., styrene-ethylene-butylene-styrene polymer.
Alternatively, the binder may be EPDM rubber, a PVDF homopolymer, or a
linear tri-block polymer cross-linked with a melamine resin, e.g.,
styrene-ethylene-butylene-styrene cross-linked with a melamine resin. One
or both sides of the current collector, which may consist essentially of
aluminum, can be coated with active cathode material.

[0009] In another aspect, the invention features a flexible cathode for a
lithium battery. The cathode includes: (a) a current collector including
aluminum foil; and (b) active cathode material including: (i) manganese
dioxide; (ii) conductive materials; and (iii) a binder.

[0010] In another aspect, the invention features a method for making a
cathode for a lithium battery. The battery includes: (a) combining a
catalyst, conductive materials, a solvent, and a binder to form a
mixture; (b) dispersing the mixture to form a slurry; (c) applying the
slurry to a substrate using a solution extrusion process to form a coated
substrate; and (d) drying the coated substrate. The battery can further
include calendering the cathode after step (d).

[0011] The substrate can be an aluminum foil, which can be flame treated
and coated with a primer prior to step (c). The solvent can be a
hydrocarbon solvent, e.g., a paraffinic solvent or an aromatic
hydrocarbon solvent.

[0012] The details of one or more embodiments of the invention are set
forth in the accompanying drawings and the description below. Other
features, objects, and advantages of the invention will be apparent from
the description and drawings, and from the claims.

[0015] An electrochemical cell, such as the prismatic cell 10 shown in
FIG. 1, includes an anode in electrical contact with a negative lead, a
cathode in electrical contact with a positive lead, a separator, and an
electrolytic solution. The anode, cathode, separator, and the
electrolytic solution are contained within a case. The electrolytic
solution includes a solvent system and a salt that is at least partially
dissolved in the solvent system.

[0016] The cathode includes an active cathode material. The active
material can be, e.g., a metal oxide such as MnO2. Electrolytic
manganese dioxide (EMD) is preferred. Other active materials are
described in Blasi et al., U.S. Ser. No. 10/022,289, filed Dec. 14, 2001,
which is herein incorporated by reference in its entirety. For example,
the active material can be iron disulfide. The cathode also includes
conductive materials such as carbon black and graphite.

[0017] The cathode also includes a binder. It is desirable that the binder
be mechanically, thermally, and chemically stable. Examples of binders
that can be used include linear di- and tri-block polymers, preferably
with no double bonds, or with conjugated double bonds, in the main
polymer chain. The binder preferably includes 29 to 33% polystyrene.
Other examples include linear tri-block polymers cross-linked with
melamine resin; ethylene-propylene copolymers with an ethylene content of
at least about 40%; ethylene-propylene-diene terpolymers with ethylene
contents below about 70%; tri-block fluorinated thermoplastics (e.g.,
TFE/HFP/VF2 terpolymer); hydrogenated nitrile rubber with at least about
30% acrylonitrile; fluoro-ethylene-vinyl ether copolymers; thermoplastic
polyurethanes (TPU); thermoplastic olefins (TPO); and PVDF homopolymers
with molecular weights around 0.5M.

[0018] The binders may be modified to improve properties of the cathode.
For example, cross linking or vulcanizing low molecular weight rubber can
considerably improve the solvent resistance of the cathode. Actual cross
linking can take place in the dryer, during the coating process.

[0020] Because binders other than polytetrafluoroethylene (PTFE) are used,
fibrillation is not necessary to achieve a flexible cathode with good
cohesion. Furthermore, extrusion processes with relatively high shear
rates can be used, because the risk of fibrillation-related thickness is
minimized.

[0021] The active material, conductive materials, and binder are combined
with a solvent or solvents to form a slurry. In formulating the slurry,
the interaction between the binder solution and the active powders (e.g.,
the manganese dioxide, the carbon black, and the graphite) must be
considered. The solvent determines the application rheology for the
coating process; solvents are selected to promote defect-free and uniform
drying of the cathode. The solvents can also serve as a fugitive
plasticizers or latent solvents to control drying.

[0022] Preferred solvents include normal and branched hydrocarbons, such
as hexane; iso- and cyclic paraffinic solvents such as VM&P Naphtha HT;
and aromatic hydrocarbon solvents such as Shell Sol A100. Other
hydrocarbon solvents may be used as well. Blends of the solvents may be
used as well. For example, a blend may contain 40% by weight of an
aromatic hydrocarbon blend; 30% by weight of iso- and cyclic paraffins;
and 30% by weight hexane.

[0023] A typical slurry formulation contains 1-10%, preferably 2-5%, by
weight binder, 50-80%, preferably 60 to 70%, by weight active powders,
and 25-40%, preferably 30 to 35%, by weight of solvent(s). On a dry
basis, the cathode preferably contains less than about 3% binder by
weight, and more than about 97% of the active powders. The slurry solids
are preferably 65-75% by weight, and the viscosity range of the slurry is
from 25,000 to 45,000 cps. Table 1 shows some typical cathode
formulations.

[0024] The cathode also contains a current collector. The current
collector is generally an aluminum alloy, e.g., aluminum foil. The type
of foil to be used will depend on the equipment used to coat the foil and
wind the electrodes. Examples of foils that can be used include alloy
#1145, temper H19 at 1.0 mil (0.001 inch) thick, and temper H0 at 1.5 mil
thick. The foil can be flame-treated or corona-treated to improve
wettability. Both methods can increase the surface energy of foil from 35
Dyne/cm to 68-70 Dyne/cm. A primer can then be applied. Alternatively,
foils with primers already applied can be purchased. For example,
pre-primed foils can be purchased from Lamart Corp. A preferred current
collector is aluminum foil that has been primed with the commercially
available water-based primer (Acheson EB 012). The water based primer can
be applied using spray, gravure, and intermittent reverse roll coating
techniques. The coating weight is preferably 0.5 to 1.0 mg/cm2.

[0025] The first step in forming the cathode is to disperse the powders in
the binder solution. Slurry formulations can be dispersed using either a
ball mill or planetary mixer for bench scale processes (e.g., batch size
0.75 kg), and a Henshel mixer FM 10 for scaled-up processes (e.g., batch
size 8 kg). The dispersion time can be between about 0.5 and 1.5 hours.
The degree of dispersion is measured with a Hegman gauge. The slurry
density is preferably about 1.8-1.9 g/cc; the slurry is preferably about
73-75% by weight solids; the viscosity is preferably about 350-500 P at
10 sec-1 at 75° F. The viscosity is measured using a
Brookfield DV III, 50 rpm, spindle 7. The slurries made with these
dispersion methods can be stable for at least 5 days; some are still
usable after eight weeks.

[0026] The next step in making the cathodes is to coat the aluminum foil
current collector with the slurry. This can be done using a closed,
pressurized fluid dispensing system. Referring to FIG. 2, the slurry is
pumped into pressure pot 30. Air 31 is pumped into the pot, forcing the
slurry through slurry feed line 32. From the slurry feed line, the slurry
enters metering pump 34. The metering pump regulates the flow of solution
through feed line 36. Line 36 feeds into extrusion die 38. Foil 40, which
has been treated as described above, moves over backing roll 42. As the
foil passes by the extrusion die, the cathode slurry is applied to the
foil. The gap between the extrusion die and the backing roll determines
the wet thickness of the coating. The current collector can be coated on
one side, or on both sides. For example, the gap between the extrusion
die and the backing roll can be set at 14-16 mil for the first pass of
the foil. If the foil is 1 mil thick, this setting will result in a
coating on one side of the foil of about 7-10 mil, when dry. If the other
side is to be coated, the gap between the backing roll and the extrusion
die can be set to 23-25 mil. This will result in a current collector in
which each side has a coating with a thickness of about 7-10 mil when
dry. Since the foil has a thickness of about 1 mil, and each side has a
layer of primer about 0.5 mil thick, the total thickness of the dry
cathode is 16-22 mil.

[0027] A lab coater with a 4-inch wide web can be used. The speed of the
backing roller can be set to yield a line speed of 19 cm/minute. A
reverse comma coating technique can be used. The basis weight of the dry
cathode is optimally 45 to 50 mg/cm2 per side. Solution extrusion
methods are further described in Modern Coating and Drying Technology (E.
Cohen and E. Gutoff, eds., 1992) and Walter Michaeli, Extrusion Dies (2d
rev. ed. 2000).

[0028] After coating, the cathode is dried by passing through zones in
which heated air is directed at the wet surface of the cathode. The air
speed and temperature are gradually ramped from zone to zone. Exemplary
temperatures are 45-80° C. and 70-130° C. for zones 1 and
2, respectively. If the cathode is dried too quickly in the first zone,
it can be prone to cracking Exemplary coating and drying process
parameters are shown in Table 2.

[0029] Typically, bench-coated cathodes are considered to pass the drying
test if the coating is uniform and defect-free after 15 seconds at room
temperature and 3 minutes at 100° C. Analytical tests indicate
that no more than 0.5% of residual toluene is present in the dry cathode
after this drying schedule.

[0030] After drying, the cathode is calendered. Before calendering, the
uncoated edges of the cathode are slit off to avoid wrinkling of the
coating-free zones. The cathode can be calendered using a 4-roll modified
"Z" calender with a roll width of 12 inches and a roll diameter of 16
inches. The rolls may be heated or cooled as needed. The cathode is
preferably calendered off-line, in a continuous mode (e.g.,
reel-to-reel). A 2×2 roll configuration with two nips, or a 2 roll
configuration with one nip can be used. The materials are preferably
calendered between room temperature and 60° C. A line speed of 3
feet/minute can be used.

[0031] The cathode is calendered to achieve a desired porosity. For
example, in some embodiments, a porosity of 30-35% is desired. Other
desired features for the calendered cathode include a total coating
weight of about 100 mg/cm2 for double-sided cathodes; a density of
greater than about 2.85 g/cc; and an extension of no more than about 5%,
and preferably about 1.5% to about 2.5%. For cathodes coated on one side
only, a green (i.e., dried but not calendered) cathode having a thickness
of 7-11 mil is preferably calendered to a total finished thickness of
about 6-8 mil (a coating layer of 4.5-6.5 mil, 1 mil thick foil, and a
0.5 mil primer layer).

[0032] The finished cathode can be pleated. That is, it can be bent back
on itself 180°, such that the two sides contact each other. A
cathode that consists of a foil current collector coated on one side was
folded with the foil on the outside, and the coating on the inside. After
being pleated, the cathode showed no visible cracking.

[0033] The finished cathode can also be wound, that is, wound around a
mandrel. A cathode consisting of an aluminum foil current collector
coated on both sides was wound around a 27.5 mm×0.9 mm mandrel.
Visual inspection revealed that the coating was not cracked, even after
the cathode was wound, although in some cases the foil may be cracked.
Cathodes that can be pleated or wound, as just described, are said to be
"flexible." The adhesion of the coating can also be tested using a
10×10 square cross hatch test.

[0034] The cathodes can be used in lithium cells, such as the prismatic
cell 10 shown in FIG. 1. These cells also include an anode, a separator,
an electrolyte, and a container. The anode can consist of an active anode
material, such as lithium. The separator can be formed of any of the
standard separator materials used in nonaqueous electrochemical cells.
For example, the separator can be formed of polypropylene, (e.g.,
nonwoven polypropylene or microporous polypropylene), polyethylene,
and/or a polysulfone. Separators are further described in U.S. Pat. No.
5,176,968.

[0035] The electrolyte can be in liquid, solid or gel (polymer) form. The
electrolyte can contain an organic solvent such as propylene carbonate
(PC), ethylene carbonate (EC), dimethoxyethane (DME), dioxolane (DO),
tetrahydrofuran (THF), acetonitrile (CH3CN), gamma-butyrolactone,
diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate
(EMC) dimethylsulfoxide (DMSO) methyl acetate (MA), methyl formiate (MF),
sulfolane or combinations thereof. The electrolyte can alternatively
contain an inorganic solvent such as SO2 or SOCl2. The
electrolyte also contains a lithium salt such as lithium
trifluoromethanesulfonate (LiTFS) or lithium trifluoromethanesulfonimide
(LiTFSI), or a combination thereof. Additional lithium salts, for
example, lithium iodide, that can be included are listed in U.S. Pat. No.
5,595,841, which is hereby incorporated by reference in its entirety. In
some embodiments, the electrolyte may contain LiPF6; in other
embodiments, the electrolyte is essentially free of LiPF6. The
electrolyte also contains a perchlorate salt, which inhibits corrosion in
the cell. Examples of suitable salts include lithium, barium, calcium,
aluminum, sodium, potassium, magnesium, copper, zinc, ammonium, and
tetrabutylammonium perchlorates. Generally, at least 500 ppm by weight of
the perchlorate salt is used; this ensures that there is enough salt to
suppress corrosion. In addition, less than about 20,000 by weight of the
perchlorate salt is generally used. If too much perchlorate salt is used,
the cell can be internally shorted under certain conditions during use.
The electrolyte is further described in Blasi et al., U.S. Ser. No.
10/022,289, filed Dec. 14, 2001.

[0036] To assemble the cell, a separator can be cut into pieces of a
similar size as the anode and the cathode and placed between the two. The
anode, cathode, and separator are then placed within a case, which can be
made of a metal such as nickel, nickel plated steel, stainless steel, or
aluminum, or a plastic such as polyvinyl chloride, polypropylene,
polysulfone, ABS or a polyamide. The case is then filled with the
electrolytic solution and sealed. Additional methods for assembling the
cell are described in U.S. Pat. Nos. 4,279,972; 4,401,735; and 4,526,846.
Other configurations of battery 10 can also be used, including, e.g., the
coin cell configuration.

[0037] The invention is further described in the following examples, which
do not limit the scope of the invention described in the claims.

Example 1

[0038] A cathode was prepared using the techniques described above. The
slurry included:

[0039] The slurry was used to coat both primed and unprimed aluminum foil.
Cathodes were prepared as described above, and their performance was
measured in two test vehicles (2/3A cell and coin cell). The performance
of the flexible cathodes was compared to cathodes made with stainless
steel expanded mesh current collectors. In the 2/3A cells, the
performance of the cathodes was comparable. In the coin cells, the
performance for the flexible cathode made using primed foil was
comparable to that of the cathode with a stainless steel current
collector, but the cathode made with unprimed aluminum foil had a
performance 30% below that of the cathode made with the stainless steel
current collector. The flexible cathodes could be wound around a 0.177
inch mandrel without cracking and delamination and could be pleated.

Example 2

[0040] A cathode was prepared using the techniques described above. The
slurry contained:

[0045] The flexible cathode could be wound around a 0.177 inch mandrel
without cracking and delamination.

[0046] All publications, patents, and patent applications mentioned in
this application are herein incorporated by reference to the same extent
as if each individual publication, patent, or patent application was
specifically and individually indicated to be incorporated by reference.

Other Embodiments

[0047] A number of embodiments of the invention have been described.
Nevertheless, it will be understood that various modifications may be
made without departing from the spirit and scope of the invention. For
example, although the examples described above relate to cathodes for
primary (i.e., non-rechargeable) lithium batteries, the invention can be
used to prepare cathodes for rechargeable lithium batteries as well.
Other embodiments are within the scope of the following claims.